This invention relates to a coating apparatus for applying a liquid agent such as a resist solution, to a substrate such as a semiconductor wafer or an LCD substrate.
The photolithography technique is used in a process for manufacturing semiconductor devices. In the photolithography, a resist is coated on a semiconductor wafer, then exposed to have a predetermined pattern, and developed. As a result, a resist film of a predetermined pattern is formed on the wafer. This resist film is subjected to a further film-forming treatment and an etching treatment, thereby forming a circuit of a predetermined pattern. A series of resist coating treatments as above are performed by a coating/developing apparatus as described in U.S. Pat. No. 5,664,254.
Uniformity in the thickness of a resist film and a reduction in the thickness are now being strongly demanded to meet a recent tendency to reduce a line width, employed in semiconductor devices, to a submicron value. At present, a spin coating method is mainly used to coat a wafer with a resist. In this method, the thickness of a resist film is controlled by adjusting the rotational speed of the wafer. In order to, for example, thin the resist film, the wafer must be rotated at a high speed.
In the conventional resist coating apparatus, however, the mechanical performance of the spin chuck is limited, and hence the wafer rotational speed has an upper limit. As a result, the thickness of the resist film cannot be reduced to less than a certain value. In particular, in the case of coating a large-diameter wafer of eight inches or twelve inches, the speed of diffusion or spread of a resist solution on the wafer is low, which makes it difficult to form a thinner resist film than ever.
Moreover, immediately after changes in process conditions such as a required film thickness, the kind of resist solution, etc., a resist film coated on a first wafer may well have a thickness which is not equal to a target value. In this case, the coated resist must be removed from the wafer, and resist coating performed again.